1. Field of the Invention
The present invention pertains to the field of optics. The invention more particularly concerns an optical device used in fiber optic communication systems to combine or separate optical signals in an optical fiber and is known as an interleaver.
2. Discussion of the Background
An interleaver can be used to double the number of available channels in a WDM (wavelength division multiplexing) system. U.S. patent application Ser. No.09/952,286 discloses an interleaver design based on cascaded Mach-Zehnder interferometers (MZI). As shown in FIG. 1, micro-optic nonpolarizing beam splitting prisms 13, 14, 18, 19, 23, 24, 28, 29 and precision glass spacers 17, 22, 27 are used to construct such an interleaver 10. A dielectric beam splitting coating 15 separates prism 13 from prism 14. Likewise, dielectric beam splitting coating 20 separates prism 18 from prism 19, dielectric beam splitting coating 25 separates prism 23 from prism 25, and dielectric beam splitting coating 30 separates prism 28 from prism 29. The dielectric beam splitting coatings 15, 20 split incident light into two paths where each path has approximately fifty percent of the optical power of the incident light. The dielectric beam splitting coatings 25, 30 transmit approximately ninety-six percent of the incident light and reflects the remaining approximately four percent of the incident light.
The interleaver 10 also includes adjusting plates 16, 21, 26 all of which have an optical path length, d1. The precision glass spacer 17 has an optical path length, Lo+d1. The precision glass spacer 22 has an optical path length, 2Lo+d1. The precision glass spacer 27 has an optical path length, 4Lo+d1. A coupling lens 12 is attached to an optical fiber 11, a coupling lens 31 is attached to an optical fiber 33, and a coupling lens 32 is attached to an optical fiber 34.
The arrow adjacent to the optical fiber 11 shows the direction of the propagation of the light signal contained therein. The light signal contained within optical fiber 11 consists of many light signals of both even and odd channels. The light signal 2 emerges from the coupling lens 12 and enters the combination of prisms 13, 14 along with the dielectric beam splitting coating 15. Upon exiting the combination of prisms 13, 14 the light signal is separated into two paths 3, 4. The light traveling along path 3 travels through the adjusting plate 16 and then enters the combination of prisms 18, 19. The light traveling along path 4 travels through the precision glass spacer 17 and then enters the combination of prisms 18, 19. The light signals continue to travel through the remaining prisms 23, 24 and 28, 29 of the interleaver 10 in a similar manner until the exit the last combination of prisms 28, 29 along paths 5, 6. The even channel data of light signals travel along path 5 and into coupling lens 31 and then into optical fiber 33 in the direction of the arrow adjacent to optical fiber 33. The odd channel data of light signals travel along path 6 and into coupling lens 32 and then into optical fiber 34 in the direction of the arrow adjacent to optical fiber 34.
The design of interleaver 10 is well suited for interleaving/de-interleaving WDM channels with moderate channel spacing. For very dense channel spacing, the required thickness of the precision glass spacers 17, 22, 27 is large, which in turn leads to difficulties in alignment and a large package size. For example, assuming the refractive index of the precision glass spacer is 1.5, for 12.5 GHz channel spacing the required thickness for spaces 17, 22, and 27 in FIG. 1 is 25, 50 and 100 mm, respectively.
FIG. 2 shows a conventional planar waveguide interleaver 40 that combines a ring resonator 44 (the equivalent of a GTI interferometer) inside a MZI 45. The interleaver has an input signal which travel along optical fiber or waveguide 41. The input signal includes even and odd signals, λ1,λ2, . . . , λn, and travels in the direction of the arrow which is adjacent to the waveguide 41. The waveguide 41 enters a coupler 42 where two waveguides 49, 50 emerge from the coupler 42. The coupler 42 has a three dB loss and evenly splits the input signal traveling in waveguide 41 between waveguides 49, 50. Waveguide 50 enters an unbalanced MZI 45 which has an optical path length difference, ΔL. A waveguide 52 exits a delay element 53 of the unbalanced MZI 45. Waveguide 49 enters a coupler 43, and an optical resonator 44 is also attached to the coupler 43. The resonator 44 has an optical path length, 2ΔL. The coupler 43 has a beam splitting ratio that transmits between eighty to ninety percent of the incident light while it reflects between ten to twenty percent of the incident light. Emerging from coupler 43 is a waveguide 51. Waveguides 51, 52 enter coupler 46. In terms of performance, the coupler 46 is substantially the same as coupler 42. Waveguides 47, 48 emerge from coupler 46. One output signal travels in waveguide 47 and contains the odd channels, λ1, λ3, λ5, . . . , λodd. Another output signal travels in waveguide 48 and contains the even channels, λ2, λ4, λ6, . . . , λeven.
The drawback of this design of waveguide form 40 is that the size of the resonator 44 is limited by the bending loss of the waveguide. In order to minimize losses, waveguide bending should be keep at a radius larger than the critical bending radius of the waveguide. A typical waveguide has a minimum bending radius in the range of 10 to 30 mm. However, for a 12.5 GHz WDM interleaver, the required radius for the ring resonator 44 is only about 1.50 mm, assuming the refractive index of the waveguide is about 1.50.
Interleavers based on this structure 40 have a near square top spectrum response curve which is desirable for DWDM applications. FIG. 3 shows the spectrum response curve of such a structure 40 which is a plot of power loss measured in dBs versus wavelength measured in microns.
FIG. 4 shows a conventional micro-optic interleaver 60 that combines a GTI resonator with a Michelson interferometer. The Michelson interferometer 65 includes a beam splitting cube 66, and two one-hundred percent reflection mirrors 67, 68. The splitting cube 66 includes two prisms 73, 74. The GTI resonator includes the one-hundred prevent reflection mirror 67 and a dielectric coating 71 applied to the beam splitting cube 66, and the one-hundred percent reflection mirror 68 and a dielectric coating 72 applied to the beam splitting cube 66. The beam splitting cube 66 evenly splits an incident optical signal into two resultant optical signals. One reflection mirror 67 is positioned a distance, 2t, away from one dielectric coating 71 and the other reflection mirror 68 is positioned a distance, t, away from the other dielectric coating 72. Each dielectric coating 71, 72 transmits between eighty to ninety percent of the light incident thereon while they each reflect between ten to twenty percent of the light incident thereon. Waveguides 61, 63, and 75 are optically connected to a circulator 62. Waveguide 75 is also optically connected to a single mode fiber collimator 64. Waveguide 70 is optically connected to a single mode fiber collimator 69.
The arrows indicate the direction of propagation of the light signals within the device 60. The input signal includes even and odd signals, λ1, λ2, . . . , λn, and travels in the direction of the arrow which is adjacent to the optical fiber or waveguide 61. One output signal travels in waveguide 63 and contains the even channels, λ2, λ4, λ6, . . . , λeven, and travels in the direction of the arrow which is adjacent to the waveguide 63. Another output signal travels in waveguide 70 and contains the odd channels, λ1, λ3, λ5, . . . , λodd, and travels in the direction of the arrow which is adjacent to the waveguide 70.
Here, the GTI resonator and the Michelson interferometer provide similar functions as the ring resonator 44 and MZI interferometer 45 shown in FIG. 2. Because the spacing in the GTI resonator can be made with thicknesses ranging from a few tens of microns to a few tens of millimeters, this configuration can be used for interleaving/de-interleaving WDM signals with normal channel spacing (typically 12.5 GHz to 200 GHz). However, since both the GTI resonator and Michelson interferometer are operated in reflection mode, alignment is more demanding, and one of the de-interleaved data streams is reflected back into the input port and is then extracted using an optical circulator 62. The use of an optical circulator 62 not only increases the complexity and cost of the device, but also introduces a higher insertion loss.
Thus, there is a need for an interleaver which has narrow channel spacing, wide and flat top passband spectral response, low cross talk, and components which are easy to align relative to one another, components which are easy to manufacture, and components which are easy to assemble as compared to known interleavers.